The valve train of an internal combustion engine has often been identified as a primary source of noise which can represent a significant part of the overall noise generated by the engine. As a rule, cam-in-block (CIB), push rod actuated, valve trains are more noisy than well designed overhead cam (OHC) valve trains. This difference in noise levels between CIB and OHC valve trains is commonly attributed to differences in stiffness, since the stiffer valve train is generally found to be quieter and CIB designs are usually more compressible and thus less stiff than OHC in valve trains.
Through various tests, we have learned that stiffer valve trains will generate less noise not, as has been believed, because of fewer "no follow" noise events related to the closing of clearances but because the valves will open at lower rates of acceleration and close at lower velocities. This action is related not only to the system stiffness but also to the friction level of the valve train. High friction of the connecting elements between each cam follower and the associated valve will retard the valve opening, leading to harsher opening accelerations which may introduce vibrations that can contribute to higher closing velocities and noise.
Valve opening is further delayed by compressibility of the valve train linkage through deflections of the camshaft, valve lifter, pushrod, rocker arm stud and spring retainer. These, collectively, contribute lost motion to the valve actuation. This is because the valve does not open until the net force developed by the valve train system overcomes the effect of valve spring preload and the friction above the valve lifter (primarily rocker arm friction). The point at which the valve opens is, therefore, nearly independent of speed. In like manner, the valve closing point occurs at a cam lift which is related to the system stiffness, hysteresis and associated closing loads.
In a particular test of an automotive CIB engine with pushrod and rocker arm actuation, it was found that valve opening was delayed until the corresponding cam lift had reached a value of 0.0030 to 0.0035 inches with a corresponding rocker arm stud load of 180-200 lbs. At this point the cam acceleration was very high, approaching the maximum cam acceleration rate, so that a very harsh opening of the valve resulted. This excited a system resonance which, at higher speeds, continued through the valve cycle, due to low internal damping, and contributed to higher valve closing velocities.
At lower speeds, below 3,000 rpm, friction existing in the rocker arm pivot caused another shock load to the system, producing a resonant oscillation which continued until valve closing. Thus, the valve closing was very impulsive and consistently occurred at a cam lift of about 0.0040 to 0.0045 inches where the cam velocity remained quite high.
It is the valve closing event that is usually heard as valve train noise, but the opening of the valve and the friction reversal at maximum lift play a large role in determining how hard the valve impacts it's seat. The cam velocity at the closing point is, however, a major factor in controlling the impact energy and, hence, noise generation.
As has been previously mentioned, an OHC valve train is generally quieter than a CIB valve train because, as a rule, the system stiffness is higher and the friction level is lower. Similarly, the system vibration response in an OHC engine is generally higher in frequency and lower in amplitude. Thus, in a well designed OHC valve train, the opening and closing events are easier to define, due to lower system deflections, and the OHC cam designer may not need to account for the system's lost motion. In contrast, the CIB engine designer should account for the system's lower stiffness and higher friction which are easily determined through measurements. However, while many CIB cam profiles incorporate so called opening and closing ramps, these are not generally designed with the system deflection in mind.